Contaminants may be held both in pore water and on/in the solids fraction of soil samples. Often it is desirable to know how much contaminant could be released from the sample. Simple separation of the pore water (e. g. by a centrifuge method) will not enable us to know how much contaminant might be released from the solids by desorption and leaching. To find this information, extraction tests of some kind need to be performed in which the contaminant is encouraged to move from the solids into the liquid phase by the arrangement of the tests. This is the subject of the first part of this section. Once the liquid phase has been extracted, chemical tests can be performed on the contaminated water — this is described in the second part of this section.
Soils are complex matrices made up of numerous constituents with different and variable physical and chemical properties. Such constituents present variable capacities of interactions with pollutants, which drives the partitioning between the liquid and the solid phases. Pollutants of soils can thus be dissolved in the solution, can be adsorbed or make complexes with organic or inorganic constituents, or can be partially or totally transformed (bio-geo-chemical dynamic) (ADEME, 1999). All these forms are in relation and can, according to the type of matrix and the properties of the pollutant or external factors, induce an increase or a decrease in the mobility and (bio) availability of pollutants. These different forms can be extracted selectively from the matrix thanks to laboratory extraction methods using appropriate chemical reactants (Tessieretal., 1979; ADEME, 1999).
The environmental performance of a material is rather based on release than on total content of potentially dangerous constituents (van der Sloot & Dijkstra, 2004). Selective extraction procedures allow the assessment of the geo-chemical distribution of pollutants in the solid matrix (Colandini, 1997), and therefore the choice of extraction methods depends on the purpose of the investigation. This section essentially deals with inorganic pollutants.
Organic pollutants are often insoluble in water, though may be miscible by surfactants (e. g. detergents) or may exist in water as emulsions. Alternatively, the water and the organic chemical may be self-segregating leading to layered “oil” and other fluids, their relative positions dependent on relative density. Interaction of the organic fluid and solid is complex, depending largely on surface chemistry effects which will not be explained here (see e. g., Yong et al. (1992) for further information). Sorption of organic fluid is largely limited to organic solids.
